1. Field of the Invention
The present invention relates generally to improved reaction chambers for use in Chemical Vapor Deposition (CVD) systems, and more particularly to improvements in reaction chambers for use in epitaxial deposition systems for processing wafers on a one-at-a-time basis and for providing a more efficient deposition, a more uniform deposition on the substrate or wafer to be processed, and for reducing or eliminating deposits beneath the susceptor.
2. Description of the Prior Art
Chemical Vapor Deposition (CVD) is the formation of a stable compound on a heated substrate by the thermal reaction or decomposition of certain gaseous compounds. Epitaxial growth is a highly specific type of CVD that requires that the crystal structure of the substrate or wafer be continued through the deposited layer.
Chemical Vapor Deposition systems take many forms but the basic components of any CVD system usually include a reaction chamber which houses the wafer(s) to be processed, a gas control section, a timing and sequence control section, a heat source, and an effluent handling component. A great variety of ways of implementing each of these components leads to a great number of individual reactor configurations in prior art systems.
The purpose of the reaction chamber is to provide a controlled environment for the safe deposition of stable compounds. The chamber boundary may be quartz, stainless steel, aluminum or even a blanket of a non-reacting gas, for example, nitrogen. Commercial epitaxial deposition (epi) reaction chambers are generally classified as being one of the following three general types, depending primarily upon gas flow. Horizontal systems are employed wherein the wafers are placed horizontally on a boat or susceptor and the gas flows horizontally in one end of the reaction chamber, across the wafers, and out the other end. In vertical systems, the wafers are placed horizontally on a susceptor with the gas flow vertically toward the wafers from the top and the susceptor is normally rotated to provide more uniform temperature and gas distributions. In cylindrical or barrel reactor systems, the wafers are placed vertically on the outer surface of a cylinder, and the gases flow vertically into the chamber from the top and pass over the wafers on the susceptor which rotates for uniformity of deposition.
Heating in a cold-wall CVD system is accomplished through the use of radio frequency (RF) energy, or by radiation energy commonly in the ultraviolet (UV), visible, or infrared (IR) bands or by resistance heating. In an RF heated susceptor, the energy in an RF coil is coupled into a silicon carbide coated carbon susceptor. The wafers are heated through their contact with the susceptor. Radiant UV or IR heating is accomplished by the use of high intensity lamps that emit strongly in the ultraviolet, visible, and/or infrared spectrum. The large amounts of energy from these lamps heat the wafers and their holders by radiation. In both types of cold-wall heating, the walls of the chamber are cold, in comparison to the wafers themselves. The chamber walls must be cooled to prevent radiation from the lamps and the susceptor from producing a large temperature rise.
The reaction chamber is used in epitaxial deposition systems to provide the carefully controlled environment needed for the epitaxial deposition to take place is a critical component of the epitaxial reactor. Three basic reactor chamber configurations are used in the semiconductor processing industry including the horizontal reactor, the vertical reactor, and the barrel reactor, all of which were previously described herein.
Prior to reactor heat-up, any residual air that remains in the chamber must be removed or purged. Prior to cool-down, following the deposition cycle, any gases remaining from the growth process are flushed out.
The various gases used in an epitaxial reaction chamber include a non-reactive "purge" gas which is used at the start and end of each deposition if the reaction chamber is opened to the atmosphere after every run as is normally done. The non-reactive purge gas, usually nitrogen, is used to flush unwanted gases from the reaction chamber.
A carrier gas is used before, during, and after the actual growth cycle. The carrier gas is mixed with the gases responsible for etching, growth, or doping the silicon as each is added. Hydrogen, is most often used as a carrier gas, although helium is sometimes employed.
Etching gases may be used prior to the actual epitaxial deposition wherein etching is performed to remove a thin layer of silicon from the surface of the wafer together with any foreign matter or crystal damage that is present on it. The etching prepares atomic sites for nucleating or initiating the epitaxial deposition process.
The carrier gas is normally hydrogen. The source gases for silicon conventionally used for epitaxial depositions include Silane (SiH.sub.4); Dichlorosilane (SiH.sub.2 Cl.sub.2); Trichlorosilane (SiHCl.sub.3); and Silicon tetrachloride (SiCl.sub.4). The dopant gases normally used in epitaxial deposition include Arsine (AsH.sub.3); Phosphine (PH.sub.3); and Diborane (B.sub.2 H.sub.6). The etching gas is commonly HCl.
The problems inherent in all prior art systems of Chemical Vapor Deposition, and more particularly in the epitaxial deposition systems, include the non-uniform deposition on the surface of the wafer to be processed; the presence of contaminants in the reaction chamber prior to processing; wall deposits formed on the interior walls of the reactor chamber; deposition of the reactant chemicals on the heated susceptor and its support structure; inefficient gas flow characteristics; slow processing times; and non-uniform depositions due to uncontrolled gas velocity profiles or gas density profiles.
These problems become even more important with the modern trend away from batch processing systems toward single wafer or one-substrate-at-a-time processes. In a single wafer-at-a-time processing system, the same volume of gas normally flowing through a reaction chamber with many wafers to be processed cannot be used since too much reactant gas will be consumed for one wafer. Still further, the cycle times to process a batch of wafers in a conventional batch processing system are far too long for single wafer processing. A single wafer process requires a more rapid deposition rate to minimize the cycle time. Within a single wafer system, the deposits form reaction by-products build-up far more rapidly on a per wafer basis than in batch processing systems. Customers are increasingly demanding reduced particulate contamination. As a result, these deposits must be controlled or minimized in order to reduce the particulate contamination.